Fix evaluation of constant map with duplicate keys.

pkg/compiler/lib/src/constants/expressions.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.constants.expressions;
 
 import '../common.dart';
 import '../constants/constant_system.dart';
 import '../core_types.dart';
 import '../dart_types.dart';
@@ skipping 464 matching lines @@
       keys[index]._createStructuredText(sb);
       sb.write('->');
       values[index]._createStructuredText(sb);
     }
     sb.write('])');
   }
 
   @override
   ConstantValue evaluate(
       Environment environment, ConstantSystem constantSystem) {
+    Map<ConstantValue, ConstantValue> valueMap =
+        <ConstantValue, ConstantValue>{};
+    for (int index = 0; index < keys.length; index++) {
+      ConstantValue key =
+          keys[index].evaluate(environment, constantSystem);
+      ConstantValue value =
+          values[index].evaluate(environment, constantSystem);
+      valueMap[key] = value;
+    }
     return constantSystem.createMap(
         environment.compiler,
         type,
-        keys.map((k) => k.evaluate(environment, constantSystem)).toList(),
-        values.map((v) => v.evaluate(environment, constantSystem)).toList());
+        valueMap.keys.toList(),
+        valueMap.values.toList());
   }
 
   ConstantExpression apply(NormalizedArguments arguments) {
     return new MapConstantExpression(
         type,
         keys.map((k) => k.apply(arguments)).toList(),
         values.map((v) => v.apply(arguments)).toList());
   }
 
   @override
@@ skipping 1382 matching lines @@
     visit(exp.name);
     if (exp.defaultValue != null) {
       sb.write(', defaultValue: ');
       visit(exp.defaultValue);
     }
     sb.write(')');
   }
 
   String toString() => sb.toString();
 }